The present invention relates to means for providing safety and reliability to electric circuits. More particularly, the invention relates to a method and apparatus for providing an early detection and alert of an arcing fault in electrical systems that are used in residences, commercial areas, industries, automotive vehicles, etc.
Automatic and continuous protection of electric supply systems is accomplished today, by: a) fuses and circuit breakers that protect against dangerous over-currents by terminating the current flow in the event of a short-circuit or an overload; and b) ground fault interrupters which terminate the current flow whenever there is a dangerous leakage of current from the xe2x80x9clinexe2x80x9d Chase) conductor to the xe2x80x9cgroundxe2x80x9d conductor.
Another kind of electrical fault is the arcing fault which appears when two conductors in the circuit become separated by a small air gap that allows current to pass through it. In most instances of arcing fault, the current flows intermittently, as if there is a switch that is turned alternately and irregularly xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d.
Arcing fault poses the following threats and problems:
1) A fire hazard resulting from excessive heat that is liberated unexpectedly at an insufficiently protected arcing zone.
2) An arcing fault which develops into a major fault, damaging the electric appliance or machine in which it occurs and leading to a final short circuit and current termination.
3) An arcing fault which leads to bursts of over currents and to irregular current supply, which accelerates the wear of the electric appliances involved.
A series arcing fault is not protected by the above-mentioned protective means, since the current is limited by the load resistance, and is consequently below the threshold value predefined as dangerous. A parallel arcing fault is also not completely protected, since as long as the average current is below the predefined threshold value, it continues to constitute a fire hazard. Also, there is the potential for a short-circuit build-up, followed by current termination.
The need for an early warning device against the build-up of arcing conditions is clear, as such a device may be helpful in preventing fires, uncontrolled current termination, and damage to electrical appliances and machines.
In the last two decades, many devices have been introduced for the detection of arcing fault.
Most of them use the fact that discontinuities in the conduction parameters of an electric circuit lead to fast transients, and sense the resulting fast current oscillations. These high-frequency oscillations are then analyzed, by utilizing various features that differentiate between an arcing fault event and an event caused by a valid use of a load of the electric network. To facilitate the analysis, the transients are commonly converted to digital data or to other simple signals. All these suggestions differ principally, by the method of discrimination between the various events; their capabilities and efficiencies are derived accordingly.
For example, U.S. Pat. No. 4,466,071 (Russell et al.), issued in August 1984, is directed to a method and system for detecting high impedance arcing faults. The detection xe2x80x9cis realized by monitoring the high frequency components of the alternating current and evaluating the high frequency components of each cyclexe2x80x9d. The occurrence of a significant increase in the magnitude of the high frequency components signifies either an arcing fault or a normal switching operation. The duration of time over which this increase exists, discriminates between the various possibilities and can determine the presence of an arcing fault.
The method of U.S. Pat. No. 4,466,071 poses three particular drawbacks.
1. The switching of a dimmer might be erroneously identified by this system as an arcing fault event, as such can generate a high level of high frequency current components for a sufficient time.
2. If the minimal period for determining an arcing fault is too short, for example, less than 0.5 second, then a long switching event like pressing or releasing the trigger of an electric drill might be identified as a fault; and
3. If, on the other hand, the minimal period that defines an arcing fault is too long, for example, more than 0.5 second, then short signals which characterize the early stages of the evolution towards a fault might be ignored. Thus, the opportunity for providing an early warning would be lost.
U.S. Pat. No. 5,682,101 (Brooks et al.) issued in October 1997, discloses a detector that monitors the rate-of-change of the current in the line and produces a signal which is proportional to it. The detector produces a pulse whenever the rate-of-change signal exceeds a certain threshold. Each high-frequency disturbance in the current produces steep oscillations which are filtered into a selected high-frequency band, and are transformed by the detector to pulses. The pulses are xe2x80x9ccountedxe2x80x9d by charging a capacitor by an amount that is proportional to the number of the pulses. Upon exceeding a selected charge level, the detector signifies an arcing fault. The time constant for the discharge of the capacitor, in the example given therein, is 33 msec.
About the discrimination between various similar phenomena, Brooks states as follows: xe2x80x9cThe pattern in the rate-of-change signal produced by the sensor 21 indicates whether the condition of the circuit is a normal load, a normal switching event . . . or an arcing fault eventxe2x80x9d. But though this detector covers a wide range of events, still, a long switching event might load the capacitor beyond the selected charge level, and cause a false alarm.
Other drawbacks of the Brooks et al. system are: 1) the use of a blocking filter for each load in order to diminish spurious noise, which also diminishes the protected portion of the circuit; and 2) the discharge of the capacitor deletes valuable information, about suspicious events, that might enable an early detection of evolving problems.
U.S. Pat. No. 5,726,577 (Engel), issued in March 1998, discloses a detector for series arcs in AC circuits. This detector generates signals representing the second derivative of the current, which signals contain pulses in response to discontinuities in the current. The continuity properties of arcing current are very regular at currents too low to blow the arc apart; indeed, xe2x80x9cthe current is more continuous, except for discontinuities at current zero crossingsxe2x80x9d. This leads to a unique pattern of pulses, wherein a pair of pulses of opposite polarity appears, each half a cycle. On the other hand, the signal from a dimmer has a triple pulse of alternating polarities which also appears once per each half a cycle. This difference is the basis for the discrimination between pulses due to arcing and other pulses due to normal loads. However, some restrictions and drawbacks must be mentioned: This method is appropriate only for series arcing with quite low levels of currents which enable a steady and persistent arcing. But it is less appropriate for casual arcing or intermittent arcing, which have irregular behavior, with the consequence that the pair pattern is not repeated. Also, the method of U.S. Pat. No. 5,726,577 is targeted for just one dimmer and for detecting faults only in AC circuits.
U.S. Pat. No. 5,818,237 (Zuercher et al.), issued in October 1998, discloses a detector for arcing faults. It tracks the envelope of the current signal, and differentiates it. Thus a di/dt signal is generated that contains pulses in response to step increases in the current. The detector analyzes the pulses and rejects those that appear at a rate equal to, or higher than, the frequency of the power supply. In this way, the detector succeeds in eliminating pulses produced by loads such as dimmers; while the other pulses are counted, and after a predetermined number is attained within a predefined time interval, the detector actuates the current breaker. This detector cannot discriminate between signals of a true arcing fault and those of a normal load, since both contain pulses of random time spacing. For example, signals produced by a programmed washing machine, a prolonged switch operation, or other abused switch operation, contain pulses that are separated enough to be counted; the accumulation might be quite rapid, thus leading to a false alarm.
Other prior art fault detectors monitor and analyze high frequency noise. For example, U.S. Pat. No. 5,729,145 (Blades), issued in March 1998, detects arcing xe2x80x9cby monitoring the power waveform for wide band high frequency noise, and examining the detected noise for patterns of variation in its amplitude synchronized to the power waveformxe2x80x9d. This works according to the notion that each time the AC voltage across the air gap falls below the arcing threshold voltage, the arc is quenched, and RF noise ceases. These time gaps appear in synchronization with the power waveform.
Blades"" method does not account for the short (less than half-periods) or irregular bursts of sparks that often characterize initial stages of fault development. Consequently, the alert according to Blades"" method is given at a relatively late stage of the arc build-up, which might be hours or even days after the initial stages. Also, this method is intended only for AC power systems.
U.S. Pat. No. 5,835,321 (Ems et al.), issued in November 1998, discloses a detector that looks xe2x80x9cfor the presence of low frequency noise for a selectable number of cycles of the AC currentxe2x80x9d. But since switching operations usually create broad band noise and the switching events are not identified and are not excluded, some of them, especially those due to long switching operations, accumulate and lead to false alarms.
The limitations and deficiencies of the above and of other prior art detectors and of less recent models show that there is still a need for a detector that effectively and reliably discriminates between arcing signals and signals from valid sources. Specifically, there is a need for a detector that is capable of providing early warning of an incipient arcing fault.
It is an object of the present invention to provide a method and an apparatus for detecting an arcing fault in both AC and DC electrical systems.
It is another object of the invention to provide a warning of improper conduction or insulation conditions at an early stage of development.
It is still another object of the present invention to discriminate between a true arcing fault and the normal operation of switches, loads and dimmers.
To achieve the above purposes, the detector of the invention monitors the discontinuities in the current flowing in the electrical line, while converting the corresponding fast transients into discrete pulses, which are then analyzed in the following steps.
1) Dimmers are identified by their producing signals of equal time-spacing, which usually is either a half or a full period of the power supply. The corresponding pulses are then omitted.
2) Other equally time-spaced pulses, like those of a relaxation oscillator, night be identified by looking for an equal time interval between consecutive pulses or between the each other pulses. The matching pulses are then also omitted.
3) Normal switching operations, dangerous arcing processes and other arcing activity, are identified and discriminated along the following two categories: The category of undeveloped arcs (i.e. sparks or intermittent arcs), and the category of developed arcs:
i) The category of undeveloped arcs;
As a result of a normal switching operation the current amplitude is definitely changed, while after an event of arcing fault the current amplitude sometimes changes and sometimes retains its previous value. Therefore, a zero change in the current following an RF event, suggests that improper electrical condition is being formed, though, not necessarily dangerous. Such arcing events, to be called hereinafter as faulty events, are accumulated for the sake of further assessment of the situation severity.
Long events are considered to belong to the next category.
ii) The category of developed arcs:
This category deals with RF events that are longer than a predefined period called T-ARC, which is typically taken to be 2 seconds. Such events are call arcing states.
Long RF events characterize an arcing process, and are rarely due to a switching operation. Therefore, since dimmer and pulsed circuit events have already been excluded out, it is probable that a sufficiently long event signifies the existence of an arcing fault. Again, for the sake of obtaining a good statistics before concluding the exsitence of a problem, the durations of long events are accumulated. As a matter of convenience, to each arcing state, there is attached a number of equivalent arcing events, that is equal to duration of the arcing state expressed in seconds.
The counting in each category is restarted once a day or so, according to the required sensitivity.
Before ending the analysis of the pulses two points should be mentioned:
I) Electromagnetic interference scarcely infuence the detector because of the strong damping effect along the power lines. The detector itself, on being closed in an electric board, is usually far enough from e.m. sources. Also it can be shielded whenever necessary.
II) Electric machines generate e.m. noise which contains a lot of RF components. However their amplitudes are usually small. Consequently, an appropriate threshold can be chosen so as to avoid the conversion of the noise into pulses
More particularly, the invention relates to method for the early detection of faults in an electric supply system, which comprises the steps of (a) Continuously checking the existence of RF transients on the electric supply system line; (b) Transforming detected transients whose amplitude is above a predetermined level, into a sequence of pulses; (c) Analyzing the temporal spacing between pulses to identify those which are due to dimmers, and disregarding them; (d) Identifying short-noisy states structures in said sequence, states whose duration is shorter than a period T-ARC. (e) Determining short-noisy states where the current amplitudes before and after said states are different, and disregarding these states considered to be caused by normal switching operations; (i) Regarding each of the remaining short-noisy states as a faulty event and accumulating such events in a first register; (g) Identifying separately long-noisy states in said sequence, states whose duration is longer than a period T-ARC; (h) Ascribing a number of arcing events to each long-noisy state proportional to its duration, and accumulating these numbers in a second register; (i) Summing the values in said first and second registers, and if the sum exceeds a predetermined value N-ALARM, within a predetermined period, activating an alarm and/or a current breaker;
According to a more preferred embodiment of the invention, the summing step comprises the steps of: (a) Actuating an alarm and/or a current breaker if the accumulated number of the arcing events exceeds a predetermined value N-ALARM, within a predetermined period; (b) Activating a warning sign if the accumulated number of faulty events exceeds a predetermined value N-WARNING, within a predetermined period.
Preferably, in said more preferred embodiment N-ALARM and N-WARNING are separately defined according to the required detection sensitivity and the electric environment.
Preferably, the method also comprises the step of actuating an alarm or/and interrupting the current upon detecting of a sequence of pulses, exceeding a predetermined period of T-LONG-ARC.
Preferably, N-ALARM is predefined according to the required detection sensitivity and according to the electric environment.
Preferably, the identification of pulses due to dimmers is accomplished by (a) Determining pulses due to a dimmer by detecting a time spacing between two consecutive pulses, that equals a full or half a cycle; (b) Determining pulses due to n dimmers by detecting a time interval between a pulse and n pulses later that equals a full or half a cycle;
Preferably, the identification of pulses due to a valid switching is carried out by determining a following change in the current amplitude in the electrical line, which exceeds a predetermined value I-MIN;
Preferably, a sequence of pulses is defined as a noisy state provided that any two consecutive pulses constituting the state are separated by less than a duration T-QUIET.
Preferably, the determination of a following change in the current, is based on the following four perceptions:
i. After the passage of a spark, as well as when a short arc is over, there is a considerable probability that the current will return to its previous value.
ii. The period before this return takes place, is random and might as well be much shorter than one second.
iii. The statistical expectation of the pause between two normal switching operations, in an everyday circuit, is greater than one second.
iv. After a switch is turned ON or OFF the current is definitely changed, while quite adversely, after a faulty event the current will sometimes be changed and sometimes will remain unchanged.
Preferably, T-LONG-ARC is in the range of 3 seconds up to 50 seconds, and T-ARC is in the range of between 0.7 seconds and 3 seconds.
Preferably, T-QUIET is in the range of between 0.1 to seconds and 1.0 second.
The invention also relates to an apparatus for detecting faults in an electric supply system, which comprises:
a. Means for sensing RF transients on the electrical line;
b. Means for transforming the RF transients into a sequence of pulses;
c. Means for analyzing the pulses, and discarding those, if any, that are found to be originated from dimmers and those which are due to valid switching.
d. Means for further analyzing the sequence of the rest of the pulses to find according to their duration a number of arcing events or a faulty event;
e. Means for counting faulty events and arcing events; and
f. Means for alerting when the sum of detected faulty events and arcing events exceeds a predetermined number N-ALARM
Preferably, the means for finding a valid switching comprises a current measuring device for detecting a current change in the electrical line that follows RF transients on the line.